Microporous membranes are useful as battery separator film (“BSF”) for primary and secondary batteries. Such batteries include lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, etc. Improving BSF properties can lessen the risk of battery failure, particularly in lithium ion batteries.
One battery failure mode involves the softening and loss of mechanical integrity that is observed when the BSF is exposed to a temperature above the BSF's meltdown temperature. This situation might occur, e.g., when an internal short circuit converts a portion of the battery's electrical energy into heat or when the battery is exposed to an external heat source. The reduced strength of the softened BSF increases the risk of anode-cathode contact, which might lead to uncontrolled battery failure. To lessen this risk, microporous membranes have been produced with increased meltdown temperature. Japanese Patent Applications No. JP59-196706A and JP61-227804A, for example, disclose the use of polymethylpentene (PMP) to increase membrane meltdown temperature for improved battery safety.
Another battery failure mode results from increased battery temperature as electrolytic activity continues in the battery during overcharge or rapid-discharge conditions. To lessen this risk, microporous polymeric membranes have been produced as BSFs with a failsafe property called shutdown. When the membrane is exposed to a temperature above its shutdown temperature, increased polymer mobility reduces membrane permeability. This leads to reduced battery electrolyte transport, thereby diminishing the amount of heat generated in the battery. BSFs having a lower shutdown temperature are desired for improved battery safety.
Yet another battery failure mode involves the shrinkage of the BSF at elevated temperature (heat shrink), e.g., at a temperature between the BSFs shutdown and meltdown temperatures. Should heat shrinkage lead to a reduced BSF width, the close spacing between anode, cathode, and separator can lead to an internal short circuit in the battery, even at temperatures below the BSFs meltdown temperature. This is particularly the case in prismatic and cylindrical batteries, where even a small change in membrane width can result in anode-cathode contact at or near the battery's edges. To take better advantage of increased battery safety margin provided by increased BSF meltdown temperature, it is desired to lessen the amount of BSF heat shrinkage, particularly at temperatures significantly above the BSF's shutdown temperature. In particular, it is desirable to increase the BSF's meltdown temperature, decrease its shutdown temperature, and decrease its heat shrinkage, without significantly degrading other important BSF properties, such as permeability or strength.
The prior art discloses at least two ways for lowering BSF shutdown temperature. The first, disclosed in U.S. Patent Application Publication No. 2009/011745, involves utilizing ultra high molecular weight polyethylene and a second polyethylene having a relatively high terminal unsaturation content.
The second method for decreasing shutdown temperature, which is disclosed in Japanese Patent Publication No. JP2008-080536, utilizes a low melting point polymer to achieve a low membrane shutdown temperature. To avoid degrading permeability, membrane stretching occurs at a relatively low temperature of 95° C. (with no heat setting step) to avoid melting the low melting point polymer. Although there is a significant shutdown temperature improvement, the low stretching temperature and lack of a heat setting step leads to increased BSF heat shrinkage.
While improvements have been made, membranes are desired that have high meltdown temperature, low shutdown temperature, and resistance to heat shrinkage at elevated temperature.